Effect of Dehydration on the Formation of Levofloxacin Pseudopolymorphs

Differential scanning calorimetry (DSC) curves of levofloxacin hemihydrate measured under various conditions showed different thermograms. These phenomena were attributed to be the dehydration. Dehydration caused a multiple-phase transition. Dehydration at a higher temperature (above 70°C) gave a sharp endothermic peak on the DSC curve due to the melting of the γ form, and at a lower temperature (below 50°C) gave a sharp endothermic peak due to the melting of the α form.In contrast, the thermal behavior of levofloxacin monohydrate was not affected by dehydration. The difference in the thermal behavior between the hemihydrate and the monohydrate might be attributed to a difference in the interaction between levofloxacin and crystal water. Observations by thermomicroscopy, the changes in powder X-ray diffraction patterns during heating, and single X-ray analysis all supported the above interpretation.